1. Field of the Invention
The present invention relates to an optical pickup apparatus performing an operation of reading out a signal recorded in an optical disc or of recording a signal in an optical disc.
2. Description of the Related Art
There is widely used an optical disc apparatus capable of performing an operation of reading out a signal or recording a signal by irradiating a signal recording layer of an optical disc with a laser light emitted from an optical pickup apparatus.
There is generally used an optical disc apparatus using an optical disc such as CD and DVD, while there has been recently developed an optical disc apparatus using an optical disc with improved recording density, i.e., an optical disc of the Blu-ray standard or the HD DVD (High Definition Digital Versatile Disc) standard.
For a laser light for the operation of reading out a signal recorded in the optical disc of the CD standard, an infrared light with a wavelength of 780 nm is used. For a laser light for the operation of reading out a signal recorded on the optical disc of the DVD standard, a red light with a wavelength of 650 nm is used.
There is provided a protective layer with a thickness of 1.2 mm on a top face of the signal recording layer of the optical disc of the CD standard, and a numerical aperture of an objective lens for use in performing the operation of reading out a signal from the signal recording layer is specified at 0.45. There is provided a protective layer with a thickness of 0.6 mm on a top face of a signal recording layer of the optical disc of the DVD standard, and a numerical aperture of an objective lens for use in performing the operation of reading out a signal from the signal recording layer is specified at 0.6.
In contrast to the above-described optical discs of the CD standard and the DVD standard, for a laser light for the operation of reading out a signal recorded on the optical disc of the Blu-ray standard or the HD DVD standard, a laser light with a shorter wavelength, a blue-violet light with a wavelength of 405 nm is used, for example.
There is provided a protective layer with a thickness of 0.1 mm on a top face of a signal recording layer of the optical disc of the Blu-ray standard, and a numerical aperture of an objective lens for use in performing the operation of reading out a signal from the signal recording layer is specified at 0.85.
On the other hand, there is provided a protective layer with a thickness of 0.6 mm on a top face of a signal recording layer of the optical disc of the HD DVD standard, and a numerical aperture of an objective lens for use in performing the operation of reading out a signal from the signal recording layer is specified at 0.65.
As described above, for the laser light for performing the operation of reading out signals recorded on the optical discs of the Blu-ray standard and the HD DVD standard, the blue-violet light with a wavelength of 405 nm can be used. Thus, by using a laser diode in common for the both optical discs, it may become possible to produce an optical pickup apparatus capable of performing the operation of reading out signals from the optical discs of these two standards.
In order to read out signals from the both optical discs, however, the numerical aperture needs to be changed corresponding to each of the optical discs since the location of the signal recording layer and the numerical aperture required of the objective lens are different between the two optical discs. There has been developed an optical pickup apparatus capable of performing the above-described operation (see Japanese Patent Application Laid-Open Publication No. 2006-172605).
Recently, there is commercialization of an optical disc apparatus capable of using not only optical discs of the above described CD standard and DVD standard but also optical discs of the Blu-ray standard or the HD DVD standard. Consequently, an optical pickup apparatus used for such an optical disc apparatus naturally results in being configured so as to be able to perform the operation of reading out the signals from the signal recording layer provided in the optical disc of a compatible standard or the operation of recording the signals onto the signal recording layer provided therein.
In such an optical pickup apparatus, it is difficult to apply the laser light having the above-described wavelength onto the signal recording layer of the optical disc by a single objective lens, thereby using two objective lenses: one objective lens for applying the laser light to the optical discs of the CD standard and the DVD standard, for example; and the other objective lens for applying the laser light to the optical disc of the Blu-ray standard, for example (See Japanese Patent Application Laid-Open Publication No. 2006-19001).
As mentioned above, the blue-violet light with a wavelength of 405 nm is used as the laser light for the operation of reading-out a signal recorded in the optical disc of the Blu-ray standard or the HD DVD standard, however, intensity of the laser light on a disc face required for recording a signal in the optical disc in the optical disc device is remarkably larger as compared with the intensity of the laser light on the disc face required for reproduction.
In the optical disc device, a switching operation between the laser intensity for the recording operation and the laser intensity for the reproduction operation is carried out in general by switching a level of a driving signal supplied to a laser diode. However, the laser diode for generating the blue-violet light has an extremely strict reproduction margin, and quantum noise is a serious problem.
In order to solve the above problem, such an art is developed in which laser light whose output capable of a recording operation is emitted from a laser diode and an intensity filter is interposed in an optical path of the laser light during the reproduction operation so that the laser light with an intensity suitable for the reproduction operation can be obtained (See Japanese Patent Application Laid-Open Publication No. 2004-272949).
In the art disclosed in Japanese Patent Application Laid-Open Publication No. 2004-272949, there is described an art using an intensity filter for changing the intensity of the laser light, however, since this art needs not only the intensity filter but also control mechanisms for electrically controlling the intensity filter and for mechanically moving the intensity filter, there is a problem that the optical pickup apparatus becomes complicated in configuration as well as expensive.